In recent years, organic electroluminescent display has emerged as the most potential display device, due to its outstanding advantages, such as all-solid configure, self-luminous mechanism, dispensing with backlight, low driving voltage, high efficiency, minimized thickness and flexibility performance. Generally, an organic electroluminescent device can be fabricated using a flexible substrate so as to achieve a flexible, light and portable final product. As well known, the electrode and luminescent materials of an organic electroluminescent device are very sensitive to oxygen and moisture, so the presence of oxygen and moisture in the device may be a major cause of shortened service life. Therefore, it is very important to employ a suitable packaging substrate and an effective packaging method to protect an organic electroluminescent device from environment effects and improve its efficiency and life.
Conventionally, the organic electroluminescent device is constructed on a hard glass substrate, which possesses a low oxygen and moisture permeability, capable of protecting the device effectively. A majority of the flexible substrates used in flexible organic electroluminescent devices are made from polymer materials, such as PC (polycarbonate), PET (polyethylene terephthalate), PEN (polyethylene naphthalate), PES (polyether sulphone), PI (polyimide). The polymer substrate features lightness, thinness, durability and flexibility. However, the polymer substrate has relatively large oxygen and moisture permeability, due to its smaller fractional free volume and larger chain segment average degree of freedom, so that it can not effectively prevent oxygen and moisture permeating into the device, leading to the service life shortened. To solve the permeation issue of the polymer substrate, one way commonly used is to form a barrier layer on the polymer substrate to prevent the permeating of oxygen and moisture. For example, Korean Patent Registration No. 0300425 discloses a polymer sheet used for an organic electroluminescent device, which comprises a plurality of barrier layers disposed thereon, WO02/065558 discloses a transparent organic silicon protective layer disposed on a transparent polymer sheet, and WO02/091064 discloses multi-blocking layers including organic layers and inorganic layers. Although one or more barrier layers formed on the polymer substrate can effectively prevent oxygen and moisture from entering, several depositing steps must be performed, which causes the process complicated and the optical and mechanical properties of the organic electroluminescent device negatively affected. Especially, when the barrier layers include inorganic layers, the inorganic layers with low flexibility can not facilitate the mechanical stress to be diffused, and therefore have a possibility of cracking, which may induce the oxygen and moisture permeating through cracks.
A sheet of metal foil has an excellent flexibility when its thickness is low to 100 μm or below. Compared with polymers, metal foils have better heat resistance and lower coefficient of thermal expansion, and particularly do not involve the oxygen and moisture permeating issue. Thus, a sheet of metal foil can be suitably used as a substrate for the flexible organic electroluminescent device. However, there is still a problem of higher electrical conductivity and larger surface roughness associated with the metal foil when using as the flexible substrate.
Besides oxygen and moisture permeating through the substrate from bottom, permeating from the periphery of the device also should be avoided. Currently, the periphery is generally packaged by sintering frit coated thereon via laser. However, frit will indurate and have no flexibility after sintering, and thereby is not applicable to a flexible displaying device.
Thus, there is still a demand for improved flexible packaging substrate and method, to protect both the bottom and the periphery of flexible organic electroluminescent device and ensure the device possesses both excellent flexibility and minimized oxygen and moisture permeability.